The invention relates to a process for manufacturing a composite metal-plastic laminated foil, in particular having outer layers made of aluminum thin strip, and also relates to a composite foil manufactured in accordance with the said process.
What is known, for example, is a triple layer metal-plastic composite having a plastic core and metal foil on both sides, and a process according to the U.S. Pat. No. 4,172,914 for manufacturing an aluminum-plastic composite foil. This process features at least one plastic foil adhesively attached to one side of an aluminum foil, this composite being used to manufacture sealable containers for sensitive contents such as foodstuffs, pharmaceuticals, chemicals and the like to provide protection for these contents from the effects of light, oxygen, water vapor, loss of aroma and external aromas.
The process for manufacturing the aluminum (Al)-plastic foil just mentioned is such that an Al-foil is bonded to at least one plastic foil, the elongation at fracture of which is 80-180% and the load bearing capacity of which, when the composite foil is stretched, is greater than that of the Al-foil. The Al is bonded to the plastic foil(s) by an adhesive intermediate layer which exhibits a shear strength of at least 0.3 N/mm.sup.2.
Also known is the use of stretched plastic foils such as e.g. biaxial oriented polypropylene foils for the manufacture of two layer Al-plastic laminates. Container parts made from such laminates can, in combination with corresponding laminates of unstretched plastic foils, be sealed to make forms of packaging the seams of which are easily peeled apart so that the packaging can be readily torn open without requiring any aid.
As only relatively soft metal strip is suitable for metal-plastic laminates with good deep drawing properties, such forms of packaging are easily damaged by manipulation during production and in distribution. This problem arises in particular with large volume light-weight containers for fluids in which the outer layers of the metal-plastic composite are made of aluminum thin strip.
To prevent damage to such containers with a weight of up to 30 kg when full, it has been shown to be practical to employ a metal-plastic composite foil which is made up of three layers viz., a so-called triplex composite having a plastic core with thin metal strips on both sides. The use of a triplex composite of the kind just mentioned for relatively large volume containers has, however, been very limited up to now because of the unsatisfactory deep drawing properties of such composite foils in deep drawing or deep drawing by elongation of the foil (stretching). It is therefore an object of the invention to develop a process for manufacturing metal-plastic composite foils and a composite foil in the form of a triplex composite having a plastic core with thin metal strip on both sides wherein the foil can be used in particular for the production of semi-rigid light-weight containers by deep drawing or deep drawing by elongation of the foil (stretching). The said containers are of the kind with a weight of up to about 30 kg when full, are particularly resistant to breakage and leakage and can be closed or fitted together using known sealing methods.